Hello,

Download all available previous year papers from internet. Now try to solve the questions. If you think any of the question is not if your course, clear  it with your teacher. For PRMO try to solve all papers and all topics covered in it. It's one of the way to qualify PRMO. After you get selected for Second level then prepare by getting  some theory books like arihants rajjev manocha or pre college mathematics. Try these books only for theory. For approach making or question solving skills try to solve question papers. They are more than enough for RMO LEVEL.For the final stage as it needs good approach as well as theory. For this try to get 'excursion in mathematics' book. This book is mentioned in hbcse official website as well and given in inmo camp as well. Try to complete this book as fast as possible and then move to question papers.

Here is the link for preparatory video that might get you some help

https://www.youtube.com/c/ansacademy?sub_confirmation=1

All the best ....